Analytical test strips for testing analytes in heterogeneous fluid samples are well known in the art and comprise various structures and materials. These test strips typically include single or multi-layered fibrous membrane devices which receive a heterogeneous fluid, such as whole blood, and undergo a color change in response to interaction with agents/reactants imbibed: into the membrane. Prior to reaching the reactants, the fluid sample is filtered to facilitate accurate testing of the analyte. For instance, a blood sample being treated for glucose levels requires the removal of red blood cells before testing the plasma. Some test strips include additional layers that provide the requisite filtering. Other test strips attempt to filter and test a sample for a suspected analyte in a single membrane. Terminiello et al., U.S. Pat. No. 4,774,192, teaches such a dry chemistry reagent system which comprises a porous anisotropic (asymmetrical) membrane having a porosity gradient from one planar surface to the other for filtering a fluid sample and includes an indicator, flow control agent, and reagent cocktail imbibed therein for initiating the chemical reaction with the fluid sample. Anisotropic membranes, however, provide inadequate filtering and can have a tendency to produce unreliable results.
Test strip devices operate by allowing the applied heterogeneous sample to migrate to a reaction site in the membrane, where the analyte of interest in the sample reacts with the imbibed agents. The results of the reaction are usually visible through a color change in the membrane. The color change may be viewed with the naked eye and measured by a visual comparison with a color chart or reading it with a reflectance meter.
Certain problems have been noted in existing analytical test strips. Some of these problems include spillage of the sample over the edges of the strip, excessive absorption, and incomplete filtering, all of which can adversely affect test integrity. Other strips, such as those disclosed in U.S. Pat. No. 3,298,789 issued to Mast and U.S. Pat. No. 3,630,957 issued to Rey et al., require the sample to remain in contact with the reagent pad for specified time and that the blood sample be either washed or wiped off the pad. In addition, conventional strips have been known to be difficult to use in terms of the proper amount of heterogeneous fluid to place on the strip. It is also difficult to properly place and/or orient strips in a corresponding meter.
U.S. Pat. No. 5,296,192 (the “'192 patent”), issued to the inventors herein, addresses some of these shortcomings noted in the background art. The '192 patent teaches a multi-layered diagnostic test strip for receiving whole blood on which a test for a suspected analyte is performed. The multi-layered test strip device comprises two outside supports, sandwiching therebetween a spreading screen, a separating layer, and a membrane. The top support has a port for receiving the sample. The spreading screen evenly distributes the sample so that it uniformly passes into the separating layer. The separating layer removes a majority of the red blood cells from the blood sample, and the membrane removes the remaining cells. The membrane is also pretreated with reagents and conditioning agents needed for the reaction and insuring a readable, reliable color generation. The '192 patent provides a strip that may be visually read with a color comparator or a reflectance meter. The instant invention provides an improved diagnostic test strip which is built in part on some of the teachings of the '192 patent, and which has additional features for further enhancing the use and reliability of diagnostic testing. These improvements are submitted as solving the above-noted problems.